Handle Duplicate URLs Properly

[jaiyankargupta]

Currently, the extension generates a new shortened URL for every request, regardless of whether the URL has already been shortened before. This can lead to multiple shortened URLs for the same original URL, which is not efficient. Implement a check to see if the URL has already been shortened and return the existing shortened URL instead of generating a new one.

Steps to Reproduce:

1. Install the extension.
2. Send a message to the extension with the same URL multiple times.
3. Observe that different shortened URLs are generated each time.

Expected Behavior:
The extension should return the same shortened URL for the same original URL if it has already been shortened.